Heavy oils, including heavy crudes, reduced crudes, residual oils from distillation processes and bitumens, are relatively low value products and unsuitable for many of the purposes for which lighter hydrocarbon products can be practically used. To exploit these materials more fully, a multiplicity of refining processes have been developed and used. These refining processes are all based upon the recognition that as the boiling point of the hydrocarbons in petroleum increases, the hydrogen:carbon ration decreases. Conceptually, therefore, heavy oil upgrading processes function either by rejecting carbon or by adding hydrogen. Typical carbon rejection processes include thermal cracking processes such as visbreaking (a mild thermal cracking) and the various coking processes including delayed coking, fluid coking and continuous coking where the degree of severity is much greater. In the coking processes, a substantial proportion of the heavy oil feed is converted to coke as a by-product of the cracking mechanism with its rejection of carbon. In a typical delayed coking process, for example, the yield of C5+ liquids may be about 55 percent of the feed by weight with rather better conversion to the desirable hydrocarbon liquids from fluid coking processes including Flexicoking™, the proprietary fluid coking process developed by Exxon Research and Engineering which uses the excess coke by-product to generate fuel gas for use in the refinery. The dominant current commercial practice is to employ the coker as a standalone unit for converting resids but coking, as a carbon rejection process, inevitably results in the formation of significant quantities of low value petroleum coke as a by-product along with some heavy fuel oil which is undesirable.
Another widely used carbon rejection process is catalytic cracking, now used almost universally in the form of Fluid Catalytic Cracking (FCC); in this process the carbon which is rejected as coke onto the cracking catalyst is consumed during oxidative regeneration of the catalyst to provide heat for the endothermic cracking of the feed with the ultimate rejection of the carbon as carbon dioxide in the regenerator effluent gas.
Hydrogen addition, on the other hand, is carried out in the hydrocracking process in which the heavy oil feed is subjected to high temperature in the presence of hydrogen under high pressures which result in cracking of the molecules in the feed with opening of the aromatic rings to permit the hydrogen to combine with the cracking fragments. The hydrocracking reactions proceed in the presence of a catalyst, typically a bifunctional material with both cracking and hydrogenation/dehydrogenation functionality. Reaction mechanisms during hydrocracking may include initial dehydrogenation to species with a higher cracking rate followed by hydrogenation as ring opening of the aromatic species proceeds.
Using FCC as a standalone process for resid conversion is not feasible because of the high metal content and excessive coke forming nature of the resid. But there is an incentive to process resid in the FCC as it can effectively utilize the spare FCC capacity brought about by the diminished gasoline demand. The current approach for upgrading resids (removing metals and reducing CCR) before sending it to a FCC is by fixed bed hydroprocessing, where resid is treated in a series of reactors (three or more) that operates at temperatures ranging between about 350 and 425° C. (about 660 to 830° F.) and a hydrogen pressure that ranges between 10,000 and 20,000 kPag (about 1450-2900 psig) making it a capital intensive process. in addition, its high hydrogen consumption leads to high operating costs and, in addition, the capability of fixed bed hydroprocessing is generally limited to resids containing up to 100 ppm metals because of their deleterious effect on catalyst life.
The highest boiling components of heavy oils typically comprise multi-ring aromatics with side chains of varying length depending on the prior processing. The bottoms products from the catalytic cracking process are typically highly aromatic, having lost most of the side chains which are removed during the cracking process. In all cases, however, heavy oils, the term used here to encompass oils of petroleum origin of high boiling point e.g. above about 540° C., and typically low API gravity below about 20, e.g. below 8 or 10, as well as non-distillable residual fractions from atmospheric and vacuum distillation as well as bitumens and tar sand heavy oils, are highly aromatic with high concentrations of aromatics with 4 fused rings or created (designated here as ≧4Rs) which are responsible for the formation of the low value coke by-product during the coking process and the need for significant amounts of high pressure hydrogen during hydrocracking.
The mass lost in coking is attributed to aromatics in the feeds that have critically large, nonvolatile, aromatic cores (≧4 fused aromatic rings, ≧4R). These same aromatics are difficult to hydrocrack down to ≦3Rs and overcracking tends to occur to form hydrocarbon gas. This does not represent an efficient use of the expensive high pressure hydrogen used in the process. The major reason for this is that ≧4Rs are electron rich and difficult to reduce further and given that hydrocracking is a reduction, there is considerable difficulty in selectively hydrocracking the ≧4Rs to species that can more easily be hydrogenated even in the presence of high pressure hydrogen and active catalysts.
A new process chemistry that converts ≧4Rs to ≦3Rs without the use of high pressure hydrogen would be of significant value to heavy oil upgrading, whether by coking or hydrocracking since it is the aromatics with the more highly fused ring systems (≧4Rs) which are refractory to both thermal and hydrogenative cracking conditions.